A typical analog phase-locked loop circuit includes a phase detector which determines the phase relationship between a reference signal and the output of the phaselocked loop, a charge pump which is responsive to the phase detector for setting a current or voltage representative of the phase relationship, a low pass filter which has a single cutoff frequency and is responsive to the current or voltage from the charge pump, and a voltage controlled oscillator which generates the output signal from the phase-locked loop circuit and which varies the frequency of the output signal in response to a control voltage from the low pass filter. A voltage regulator is usually present to provide a regulated supply voltage to the voltage controlled oscillator.
Existing voltage controlled oscillators for analog phase-locked loops have a single control voltage input that varies the frequency of the voltage controlled oscillator. If a wide frequency range is desired, then the voltage controlled oscillator is designed so that the single input has a large gain, expressed in Hertz/volt. However, the larger the gain, the more the jitter induced in the analog phase-locked loop as the charge pump increases and decreases the voltage on the loop filter capacitor.
The typical voltage regulator is implemented with p-channel transistors. As a consequence, the voltage regulator provides limited isolation from the supply voltage and operates with reduced jitter across only a limited frequency range. The gain from the supply voltage to the regulated voltage output is typically about -6 dB across a frequency range of 100-200 MHz. In addition to circuit components which directly effect regulation, the voltage regulator includes additional circuit components which support the voltage regulating components, the additional components increasing the power consumption of the voltage regulator.
It is desirable to reduce the gain of the voltage regulator, so that the voltage regulator operates with even lower jitter across the frequency range of interest. Further, it is desirable to increase the frequency range over which the voltage regulator can provide satisfactory isolation with minimal jitter. Moreover, it is desirable to reduce the use of non-regulating components, in order to reduce power consumption.
The entire voltage regulator may be implemented externally to an integrated circuit containing the analog phase-locked loop circuit. However, this necessitates the use of at least two external connection pins of the integrated circuit to bring into the integrated circuit a regulated supply voltage and associated ground from the external voltage regulator.